Abstract
Lawrence Livermore Natl. Laboratory (LLNL) has developed a one-dimensional (1D) mathematical model to simulate modified in-situ (MIS) retorting of oil shale. In this paper we discuss application of the model to commercial-scale retorting conditions. The model was tested by comparing calculated values to those measured in experimental retort runs performed at LLNL. There was generally good agreement between the calculated and observed results for oil yield, temperature profiles, and the yields of most gas species. Retorting rates were generally overestimated by as much as 10%. The model is a useful tool for design and control of retort operations and to identify and interpret observations that differ from model predictions.
The model was used to predict the results for MIS retorting on a commercial scale, focusing on larger retorts and larger shale particle sizes, focusing on larger retorts and larger shale particle sizes than could be investigated experimentally. Retort bed properties, particularly shale composition and particle size, play an important role in determining the recoverable fraction of oil. For a given shale composition, the inlet-gas properties can be selected to help control retort operations and to maximize oil yield. Extreme variations in oil shale grade that may be encountered as a function of depth can be dealt with by appropriate changes in the composition and flow rate of the inlet gas. In addition, we show that substituting oxygen diluted with steam or CO2 (for air or air diluted with steam) can make significant improvements in the heating value of the effluent gas. Finally, we demonstrate the feasibility of retorting through a substantial interval of very low-grade shale.
Introduction
LLNL has been developing technology applicable to the MIS process of extracting oil from oil shale.1,2 Our program has involved the experimental measurement of chemical reactions and reaction kinetics,3 the operation of pilot-scale retorts,4 and the development of a mathematical model of an MIS retort.5 The objective is to help establish the technical base required to evaluate and apply the MIS method on a commercial scale.
A keystone of our program is the retort model, since it represents our cumulative knowledge of the chemical and physical processes involved in oil shale retorting. The retort model has been used in planning and interpreting pilot-scale retort experiments and has successfully predicted most of the results of those experiments.4 It has also been used in developing an operating strategy for a field MIS oil shale retorting experiment.6 The principal purpose of this work is to apply the retort model to a wide range of conditions for MIS retorting, focusing on larger retorts and larger shale particle sizes than can be investigated in a laboratory experiment. Before the results of those calculations are presented, the model is discussed in terms of its content and validity.
Model Description
The LLNL retort model is a transient, 1D treatment of a packed-bed retort. In developing the model, we adopted a mechanistic approach based on fundamental chemical and physical properties rather than empirical scaling of pilot retort experiments. The model contains no arbitrarily adjustable parameters. A complete mathematical description of the model has been given elsewhere.5 The important features, therefore, are reviewed here only briefly.
Our model includes those processes believed to have the most important effects in either the hot-gas retorting mode or the forward combustion mode. The physical processes are axial convective transport of heat and mass, axial thermal dispersion, gas/solid heat transfer, intraparticle shale thermal conductivity, water vaporization and condensation, and wall heat loss. The chemical reactions within the shale particles are the release of bound water, pyrolysis of kerogen, coking of oil, pyrolysis of char, decomposition of carbonate materials, and gasification of residual organic carbon with CO2, H2O, and O2. The chemical reactions in the bulk-gas stream are the combustion and cracking of oil vapor, combustion of H2, CH4, CHx, and CO, and the water/gas shift.
The model permits axial variations of initial shale composition, particle-size distribution, and bed void fraction. It also permits time-dependent variations of the composition, flow rate, and temperature of inlet gas. The governing equations for mass and energy balance are solved numerically by a semi-implicit, finite-difference method. The results of these calculations determine the oil yield, and the composition and temperature of both the gas stream and the shale particles as a function of time and location in the retort.
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